Introduce Testem

Add Testem as an option for running tests. For example,

```JavaScript
testem -l Chrome,Firefox,Safari,PhantomJS
```

Will run tests in all these environments in a continuous-build style. The existing PhantomJS script has not been removed, since [it runs tests against packages separately instead of together](https://github.com/tildeio/htmlbars/commit/4af5862f27ca453a56e76b9f8f6b592d5e5a255f), something I'm not sure we could easily do with Testem short of changing the QUnit runner.